This application claims the benefit of the U.S. Provisional Patent Application Serial No. 60/351,844 filed Jan. 25, 2002.
The present invention relates to a probe station.
Probe stations are designed to measure the characteristics of electrical devices such as silicon wafers. Probe stations typically include a chuck that supports the electrical device while it is being probed by needles or contacts on a membrane situated above the chuck. In order to provide a controlled environment to probe the electrical device, many of today""s probe stations surround the chuck with an environmental enclosure so that temperature, humidity, etc. may be held within predetermined limits during testing. Environmental enclosures protect the device from spurious air currents that would otherwise affect measurements, and also facilitate thermal testing of electrical devices at other-than-ambient environmental conditions. Environmental conditions within the enclosure are principally controlled by a dry air ventilation system as well as a temperature element, usually located below the chuck, that heats or cools the electrical device being tested through thermal conduction.
Many probe stations also incorporate guarding and electromagnetic interference (EMI) shielding structures within or around the environmental enclosures in order to provide an electrically quiet environment, often essential during high frequency testing where electrical noise from external electromagnetic sources can hinder accurate measurement of the electrical device""s characteristics. Guarding and EMI shielding structures are well known and discussed extensively in technical literature. See, for example, an article by William Knauer entitled xe2x80x9cFixturing for Low Current/Low Voltage Parametric Testingxe2x80x9d appearing in Evaluation Engineering, November, 1990, pages 150-153.
Probe stations incorporating EMI shielding structures will usually at least partially surround the test signal with a guard signal that closely approximates the test signal, thus inhibiting electromagnetic current leakage from the test signal path to its immediately surrounding environment. Similarly, EMI shielding structures may provide a shield signal to the environmental enclosure surrounding much of the perimeter of the probe station. The environmental enclosure is typically connected to earth ground, instrumentation ground, or some other desired potential.
To provide guarding and shielding for systems of the type just described, existing probe stations may include a multistage chuck upon which the electrical device rests when being tested. The top stage of the chuck, which supports the electrical device, typically comprises a solid, electrically conductive metal plate through which the test signal may be routed. A middle stage and a bottom stage of the chuck similarly comprise solid electrically conductive plates through which a guard signal and a shield signal may be routed, respectively. In this fashion, an electrical device resting on such a multistage chuck may be both guarded and shielded from below.
FIG. 1 shows a generalized schematic of a probe station 10. The probe station 10 includes the chuck 12 that supports the electrical device 14 to be probed by the probe apparatus 16 that extends through an opening in the platen 18. An outer shield box 24 provides sufficient space for the chuck 12 to be moved laterally by a positioner 22. Because the chuck 12 may freely move within the outer shield box 24, a suspended member 26 electrically interconnected to a guard potential may be readily positioned above the chuck 12. The suspended guard member 26 defines an opening that is aligned with the opening defined by the platen 18 so that the probe apparatus 16 may extend through the guard member 26 to probe the electrical device 14. When connected to a guard signal substantially identical to the test signal provided to the probe apparatus 16, the suspended guard member 26 provides additional guarding for low noise tests. Such a design is exemplified by EP 0 505 981 B1, incorporated by reference herein.
To provide a substantially closed environment, the outer shield box 24 includes a sliding plate assembly 28 that defines a portion of the lower perimeter of the shield box 24. The sliding plate assembly 28 comprises a number of overlapping plate members. Each plate member defines a central opening 30 through which the positioner 22 may extend. Each successively higher plate member is smaller in size and also defines a smaller opening 30 through which the positioner 22 extends. The sliding plate assembly 28 is included to permit lateral movement of the positioner 22, and hence the chuck 12, while maintaining a substantially closed lower perimeter for the shield box 24.
Referring to FIG. 2, in many cases the semiconductor wafers that are tested within such a probe station are edge coupled photonics devices. Edge coupled photonics devices are normally arranged within each semiconductor wafer in orthogonal arrays of devices. Typically, the wafers are sliced in thin strips of a plurality individual optical devices, as illustrated in FIG. 3. Edge coupled photonics devices may include, for example, lasers, semiconductor optical amplifiers, optical modulators (e.g., Machzhender, electro-absorption), edge coupled photo-diodes, and passive devices. Referring to FIG. 4, many such photonics devices provide light output through one side of the device. Normally, the photonics devices receive light through the opposing side of the device from the light output. On another side of the device one or more electrical contacts are provided. In typical operation, the light provided by the device may be modulated or otherwise modified by changing the input light and/or the electrical signal to the device, or the electrical output may be modulated or otherwise modified by changing the input light. Similarly, other photonics devices are surface coupled where the electrical contact and the light output (or light input) are both on the same face of the device, as illustrated in FIG. 5. On such surface coupled photonics device is a VCSEL laser.
Referring to FIG. 6, a typical arrangement to test such photonics devices within a probe station is shown. A set of electrical probe positioners 50 are arranged on the platen to provide electrical signals to and from the device under test, as needed. In addition, one or more optical probe positioners 60 are positioned on the platen to sense the light output from the device under test or provide light to the device under test. As it may be observed, when testing devices that include both optical and electrical attributes the number of positioners may be significant thereby potentially resulting in insufficient space on the platen to effectively accommodate all the necessary positioners. In addition, the opening provided by the platen is normally relatively small so that the space available for extending the probes through the platen is limited. This limited space significantly increases the difficultly in positioning the electrical and optical probes. Similarly, the end of the optical probes typically need to be positioned within 0.10 microns in x/y/z directions which is somewhat awkward from a position on the platen above the chuck. Moreover, the angular orientation of the end portion of the optical probe likewise needs to be very accurate to couple light between the optical probe and the device under test which is similarly difficult. In many applications extreme positional and angular accuracy is needed to couple the optical waveguide or free space optical path (i.e., optical probe) to a photonics device or another optical waveguide. Moreover, during the testing of wafers the optical probes frequently tend to be out of alignment requiring manual alignment for each photonics device while probing each of the devices.
What is desired, then, is a probe station that facilitates accurate alignment of electrical and optical probes.